Exposure to chromium (Cr) compounds, which can occur through occupational exposure or the environment, is associated with a wide array of toxic effects, including serious damage to internal organs and increased incidence of certain cancers. The intracellular reductive metabolism of Cr(VI) to Cr(III), via reactive intermediates, is thought to play a key role in the cytotoxicity, mutagenicity, and carcinogenicity of Cr(VI) compounds. Studies on Cr(VI) reduction have largely focused on the mechanisms by purified chemicals or rodent tissues, but we have determined that certain aspects of the rodent studies cannot be readily extrapolated to humans. We conducted a preliminary experiment to determine if human microsomal enzymes could generate one of the reactive Cr intermediates, Cr(V). ESR spectra collected at 77 K clearly showed the production of Cr(V) (distinct signal with g value of 1.98) during the ascorbate-mediated reduction of Cr(VI). A shallow, broad signal spanning the distinct Cr(V) signal was also seen during the ascorbate-mediated reduction of Cr(V); curiously, this broad signal (peak-to-peak width of approximately 750 G) resembles that for a Cr(IV) complex reported by Luo et al. The features of this broad signal are consistent with those of a transition metal with two unpaired d electrons, although further analysis would be required to definitively conclude its true nature. Cr(V) was also clearly evident during the reduction of Cr(VI) by human hepatic microsomes. Upon expansion of the range from 4000 G to 100 G, asymmetry in the g=198 line was observed. A much smaller Cr(V) signal was seen when pre-boiled microsomes were used; this was probably due to the exposure to the NADPH-generating system for 60 min, as NADPH is a known reductant of Cr(VI) although at a much slower rate than that catalyzed by microsomal enzymes. NADPH-generating systems have been previously shown to generate small amounts of Cr(V) readily detectable by ESR. The relative Cr(V) signal intensities indicate that the signal with pre-boiled microsomes was only 43% of that with active microsomes in generating Cr(V). Although these experiments were conducted in the absence of O2, some of the Cr intensities may not represent absolute amounts of Cr(V) produced, but rather relative "steady-state" levels under the specific experimental conditions.